Glass composition, glass sheet and method for producing same, and substrate for information recording medium

ABSTRACT

The present invention provides a glass composition including, as components, in mol %: 50 to 70% SiO 2 ; 10 to 20% Al 2 O 3 ; 2 to 5% MgO; 3 to 15% CaO; 3 to 15% Li 2 O; 3 to 15% Na 2 O; and 0 to 5% K 2 O, wherein when a content of a component X in mol % is expressed as [X], [Al 2 O 3 ]— [R 2 O] is less than −0.5%, where [R 2 O] is the sum of [Li 2 O], [Na 2 O], and [K 2 O].

TECHNICAL FIELD

The present invention relates to a glass composition and particularlyrelates to a glass composition suitable for a substrate of aninformation recording medium. The present invention also relates to aglass sheet formed of the glass composition, a method for producing theglass sheet, and even a substrate for an information recording medium,the substrate including the glass sheet.

BACKGROUND ART

Larger recording capacity and shorter access time have been required ofinformation recording devices such as magnetic disks. One of the meansfor satisfying the requests is faster rotation of information recordingmedia.

However, a substrate of an information recording medium deflects byrotation. The deflection may increase resonance of the informationrecording medium with increasing rotational speed and may end up with acollision between the information recording medium and a magnetic headto cause a read error or a magnetic head crash. Because of that, it isimpossible to decrease the distance, called the flying height, between amagnetic head and an information recording medium having a conventionalsubstrate to a certain distance or less. This prevents an increase inrecording capacity.

Substrates desirably have a high elastic modulus to reduce deflectionand resonance of the substrates. Glass is known to basically have ahigher Young's modulus than that of an aluminum alloy forming analuminum substrate, though the Young's modulus of glass depends on thecomposition thereof. Patent Literatures 1 and 2, for example, discloseglasses developed to be used as substrates of information recordingmediums.

CITATION LIST Patent Literature

-   Patent Literature 1: JP H10-081542 A-   Patent Literature 2: WO 98/55993 A1

SUMMARY OF INVENTION Technical Problem

However, the glasses disclosed in Patent Literatures 1 and 2 have a highdensity, and thus need to be improved in view of reducing load on amotor of an information recording device and preventing breakage bydropping. Patent Literatures 1 and 2 discuss increasing the specificelastic modulus of each glass in view of preventing deflection duringrotation, the specific elastic modulus being determined by dividing theYoung's modulus by the density. Even when the density is high, a Young'smodulus high enough to compensate for the high density makes it possibleto maintain the specific elastic modulus of glass at a high level. Thus,the glasses disclosed in Patent Literatures 1 and 2 have a density ofabout 2.67 g/cm³ or more, which is not sufficiently low.

The shaping temperature and devitrification temperature of glass aredesirably in appropriate ranges to produce the glass by a method,typically a float process, suitable for mass production.

It is accordingly an object of the present invention to provide a glasscomposition having a specific elastic modulus high enough to reducedeflection caused by rotation of a substrate formed of the glasscomposition, having a low density, and suitable for mass production.

Solution to Problem

As a result of energetic studies of the contents of components in analuminosilicate glass and the properties thereof, the present inventorhas successfully achieved the above object by adjusting the contents ofglass components.

The present invention provides a glass composition including, ascomponents, in mol %:

50 to 70% SiO₂;

10 to 20% Al₂O₃;

2 to 5% MgO;

3 to 15% CaO;

3 to 15% Li₂O;

3 to 15% Na₂O; and

0 to 5% K₂O, wherein

when a content of a component X in mol % is expressed as [X], [Al₂O₃]—[R₂O] is less than −0.5%, where [R₂O] is the sum of [Li₂O], [Na₂O], and[K₂O].

The present invention also provides a glass sheet formed of the glasscomposition according to the present invention.

The present invention also provides a substrate for an informationrecording medium, the substrate including the glass sheet according tothe present invention.

The present invention also provides a method for producing a glasssheet, including:

melting glass raw materials; and

shaping the molten glass raw materials into a glass sheet by a floatprocess, wherein

the glass raw materials are prepared so that the glass sheet is formedof the glass composition according to the present invention.

Advantageous Effects of Invention

According to the present invention, a glass composition having a highspecific elastic modulus, having a low density, and suitable for massproduction can be provided.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the symbol % used to indicate the contents of components ofa glass composition is “mol %” unless otherwise specified. Preferredranges of the contents of components, the sum totals of the components,ratios determined by given expressions, property values of glass, etc.can be obtained by combining any of preferred upper and lower limitsindividually described below. Hereinafter, for simplicity ofdescription, the content of a component in mol % is sometimes expressedas [X]. The X represents a component forming a glass composition.Accordingly, for example, [Al₂O₃]+[SiO₂] means the sum of the content ofAl₂O₃ in mol % and the content of SiO₂ in mol %. [R₂O] means the sum of[Li₂O], [Na₂O], and [K₂O] ([R₂O][Li₂O]+[Na₂O]+[K₂O]), and [R′O] meansthe sum of [MgO] and [CaO] ([R′O]═[MgO]+[CaO]).

Hereinafter, the term “substantially free” is used to mean that thecontent of a component of interest is limited to less than 0.1 mol %,preferably less than 0.07 mol %, and even more preferably less than 0.05mol %. Industrially produced glass compositions often contain a smallamount of impurities derived from, for example, an industrial rawmaterial. The term “substantially” is used to mean that inevitableimpurities are allowed to be contained as long as the upper limit of thecontent thereof is as described above. Additionally, the followingdescription of embodiments of the present invention is not intended tolimit the present invention to specific embodiments.

SiO₂ is a component for forming a network structure of the glass. Silicaglass formed only of SiO₂ has a Young's modulus of about 70 GPa whilecommon soda-lime glass (the SiO₂ content is a little more than 70%) hasa Young's modulus of 72 GPa. A low SiO₂ content is generally desirableto improve the Young's modulus. The content of SiO₂ is preferably 70% orless, 68% or less, 67% or less, 65% or less, less than 64%, orparticularly less than 63%, and may be, in some cases, 62% or less, oreven 61% or less. When the Young's modulus needs to be greatly improved,the content of SiO₂ may be 60% or less or even 58% or less. Too low acontent of SiO₂ decreases the stability of the glass network structureand causes defects such as an increase in devitrification temperature.The content of SiO₂ is preferably 50% or more, 52% or more, 53% or more,54% or more, or particularly 55% or more, and may be, in some cases, 56%or more. A relatively high SiO₂ content is generally desirable todecrease the density. For a particularly low density, the content ofSiO₂ may be 58% or more.

Al₂O₃ is a component that improves the Young's modulus of the glass andthat suppresses devitrification when used in an adequate amount. Toohigh a content of Al₂O₃ increases the viscosity and devitrificationtemperature of the glass and decreases the meltability thereof. Thecontent of Al₂O₃ is preferably 20% or less, 19% or less, 18% or less, orparticularly 16% or less, and may be, in some cases, 15.5% or less, oreven 15% or less. The content of Al₂O₃ is preferably 10% or more, 11% ormore, 12% or more, or particularly 13% or more, and may be, in somecases, 14% or more.

[Al₂O₃]/([Al₂O₃]+[SiO₂]) is preferably 0.140 or more, 0.150 or more,particularly 0.160 or more, or, in some cases, 0.165 or more to increasethe Young's modulus. This ratio is more preferably 0.220 or less, lessthan 0.215, particularly 0.210 or less, or, in some cases, less than0.205 to obtain a favorable devitrification temperature.

MgO is a component that improves the Young's modulus of the glass, thatdecreases the density thereof, and that improves the meltabilitythereof. However, too high a MgO content increases the devitrificationtemperature of the glass. The content of MgO is preferably 2% or more,2.2% or more, or particularly 2.3% or more, and may be, in some cases,2.4% or more. The content of MgO is preferably 5% or less, 4% or less,or particularly 3.5% or less, and may be, in some cases, 3.3% or less,or even less than 3%.

CaO is a component that decreases the viscosity of the glass at hightemperatures and that improves the meltability thereof. However, toohigh a content of CaO is likely to cause devitrification of the glass.The content of CaO is preferably 3% or more, 4% or more, 5% or more, orparticularly 6% or more, and may be, in some cases, 7% or more, or even8% or more. The content of CaO is preferably 15% or less, 12% or less,11% or less, even less than 11%, or particularly 10% or less, and maybe, in some cases, 9% or less.

[R′O] is preferably 5% or more, 7% or more, 8% or more, particularly 9%or more, or, in some cases, 10% or more to improve the meltability ofthe glass. [R′O] is preferably 15% or less, 14% or less, particularly13% or less, in some cases, less than 12.5%, or even 12% or less todecrease the devitrification property.

[CaO]/[R′O] is preferably 0.650 or more, 0.700 or more, or particularly0.725 or more to obtain a satisfactory devitrification resistance, andis preferably 0.850 or less, 0.800 or less, or particularly 0.775 orless for the same reason.

Li₂O is a component that increases the meltability of the glass and thatis suitable for maintaining the Young's modulus among the glassnetwork-modifying components. However, too high a content of Li₂Odecreases the Young's modulus of the glass and excessively decreases theviscosity, leading to a less satisfactory devitrification resistance.The content of Li₂O is preferably 3% or more, 5% or more, 7% or more,7.5% or more, or particularly more than 7.7%, and may be, in some cases,8% or more, or even 8.2% or more. The content of Li₂O is preferably 15%or less, 12% or less, 11% or less, 10% or less, or particularly lessthan 10%, and may be, in some cases, less than 9.4%, 9% or less, or even8.8% or less.

Na₂O is a component that increases the meltability of the glass and thatsuppresses devitrification thereof. However, too high a content of Na₂Odecreases the Young's modulus and excessively decreases the viscosity.The content of Na₂O is preferably 3% or more, 5% or more, 7% or more, orparticularly 7.2% or more, and may be, in some cases, 7.5% or more. Thecontent of Na₂O is preferably 15% or less, 12% or less, 11% or less, 10%or less, or particularly less than 9%, and may be, in some cases, 8.4%or less, or even 8% or less.

K₂O is an optional component that increases the meltability of the glassand suppresses devitrification thereof when added at a small amount.However, too high a content of K₂O decreases the Young's modulus andexcessively decreases the viscosity. The content of K₂O may be 0.05% ormore, particularly 0.1% or more, or, in some cases, 0.15% or more. Thecontent of K₂O is preferably 5% or less, 3% or less, or particularly 2%or less, and may be, in some cases, 1% or less, or even 0.5% or less.

[R₂O] is desirably adjusted to 10 to 20% to keep favorable meltabilityand a satisfactory devitrification resistance of the glass and not todecrease the formability as a result of excessive decrease in viscosity.[R₂O] is more preferably 12% or more, 13% or more, particularly 15% ormore, in some cases, 15.5% or more, or even 16% or more, and isrecommended to be 20% or less, 19% or less, particularly 18% or less, insome cases, 17.4% or less, or even 17% or less.

[Na₂O]/([Na₂O]+[Li₂O]) is preferably 0.360 or more, 0.400 or more, orparticularly 0.420 or more to maintain a satisfactory devitrificationresistance, and is preferably 0.600 or less, 0.560 or less, particularly0.540 or less, or, in some cases, 0.520 or less for the same reason.

[R₂O]/([R₂O]+[R′O]) is preferably 0.550 or more, 0.560 or more, orparticularly 0.570 or more to obtain a satisfactory devitrificationresistance, and is preferably 0.635 or less, 0.630 or less, particularly0.620 or less, or, in some cases, 0.610 or less for the same reason.

The ratio of the content of Na₂O on a mass basis to the total content ofLi₂O, Na₂O, K₂O, MgO and CaO on a mass basis is preferably less than0.500 to obtain a satisfactory devitrification resistance. For the samereason, [Na₂O]/([R₂O]+[R′O]) is preferably less than 0.380.

([R₂O]+[R′O])/([R₂O]+[R′O]+[Al₂O₃]) is preferably 0.610 or more, 0.620or more, particularly 0.630 or more, or, in some cases, 0.635 or more soas not to result in an excessively low shaping temperature relative tothe absolute value of the viscosity and the devitrification temperature.This ratio is preferably 0.700 or less, less than 0.695, particularly0.690 or less, or, in some cases, less than 0.685 so as not to result inan excessively high devitrification temperature.

An increase in [Al₂O₃]— [R₂O] increases the viscosity and excessivelyincreases the devitrification temperature. [Al₂O₃]— [R₂O] is preferablyless than −0.5%, −0.7% or less, or particularly −1% or less, and may be,in some cases, −1.3% or less, −1.4% or less, or even −1.5% or less.

TiO₂ is an optional component that increases the Young's modulus of theglass. However, because TiO₂ increases the density and also affects thedevitrification property, the content of TiO₂ is preferably 0.2% orless. ZrO₂ is a similar optional component, and the content thereof ispreferably 0.2% or less. [TiO₂]+[ZrO₂] which corresponds to the sum ofthe contents of TiO₂ and ZrO₂ is also preferably 0.2% or less. Morepreferably, the glass composition is substantially free of TiO₂ and ZrO.Meanwhile, because of the decreasing effect of TiO₂ and/or ZrO₂ on adevitrification temperature TL, [TiO₂]+[ZrO₂] may be 0.005% or more.

SnO₂ and CeO₂ are optional components that can exert the refining effectas valences of Sn or Ce change. However, too high contents of SnO₂ andCeO₂ increase the density of the glass and, in some cases, lead to aless satisfactory devitrification resistance. The contents of SnO₂ andCeO₂ are each preferably 0.2% or less or even 0.07% or less, and, morepreferably, the glass composition is substantially free of SnO₂ andCeO₂. Even more preferably, the sum of the contents of SnO₂ and CeO₂ islimited to such a small value that the glass composition can beconsidered to be substantially free of SnO₂ and CeO₂. For example, thesum of the contents of SnO₂ and CeO₂ ([SnO₂]+[CeO₂]) is limited to 0.07%or less or even less than 0.07%.

SrO and BaO, which are also optional components, increase the density ofthe glass. The contents of SrO and BaO are each preferably 0.2% or less.[SrO]+[BaO] which corresponds to the sum of the contents of SrO and BaOis more preferably 0.2% or less. Even more preferably, the glasscomposition is substantially free of SrO and BaO.

Nb₂O₅, a lanthanide oxide, Y₂O₃, and Ta₂O₅ are optional components thathave an effect on improving the Young's modulus of the glass. However,these optional components sometimes increase the density of the glassand lead to a less satisfactory devitrification resistance. It ispreferred that the glass composition be substantially free of Nb₂O₅, thelanthanide oxide, Y₂O₃, and Ta₂O₅. The lanthanide oxide is an oxide ofany of the elements having atomic numbers from 57 to 71.

P₂O₅, B₂O₃, and F are optional components that accelerates melting ofthe raw materials. However, these components facilitate erosion of arefractory material of a melting furnace, and condensate on the furnacewall after volatilization and then sometimes enter a glass melt asforeign matters. It is preferred that the glass composition besubstantially free of P₂O₅, B₂O₃, and F.

It is known that addition of sulfuric acid salt as a part of a rawmaterial promotes refining. When a sulfuric acid salt is added, SO₃generated from the sulfuric acid salt is often left in the glass. Thecontent of SO₃ is preferably 0.5% or less, 0.3% or less, or even 0.2% orless. SO₃ is an optional component and the glass composition may besubstantially free of SO₃.

Examples of other optional components that can exert the refining effectinclude As₂O₅, Sb₂O₅, and Cl. However, these components have a majoreffect on the environment (the same can be said for F on this point). Itis preferred that the glass composition be substantially free of As₂O₅,Sb₂O₅, and Cl.

A typical impurity inevitably introduced from an industrial raw materialof glass is iron oxide. Iron oxide is present in the glass compositionas a divalent oxide (FeO) or a trivalent oxide (Fe₂O₃). The content[T-Fe₂O₃] of iron oxide calculated in terms of the trivalent oxide ispreferably 0.5% or less, 0.3% or less, or particularly 0.2% or less.When coloring of the glass should be avoided, it is preferred that theglass composition be substantially free of iron oxide (T-Fe₂O₃)calculated in terms of Fe₂O₃.

The glass composition according to the present invention can contain anoptional component other than the above, but it is preferred that theglass composition be substantially free of an optional component otherthan the above.

In a preferred embodiment, the glass composition according to thepresent invention has the following composition.

[SiO₂]: 55 to 64% (excluding 64%)[Al₂O₃]: 11 to 16%

[MgO]: 2 to 3.5%

[CaO]: 5 to 11% (excluding 5%)[LiO]: 7.7 to 10% (excluding 7.7% and 10%)[Na₂O]: 7 to 9% (excluding 9%)

[K₂O]: 0 to 2% [R′O]: 8 to 13% [R₂O]: 15 to 18%

[Al₂O₃]— [R₂O]: less than −0.5%

It is preferred that the following be satisfied in this embodiment.

[Al₂O₃]/([Al₂O₃]+[SiO₂]): 0.140 to 0.220([R₂O]+[R′O])/([R₂O]+[R′O]+[Al₂O₃]): 0.610 to 0.700[R₂O]/([R₂O]+[R′O]): 0.550 to 0.630

It is more preferred that the following be further satisfied in thisembodiment.

[CaO]/([CaO]+[MgO]): 0.650 to 0.850

[Na₂O]/([Na₂O]+[Li₂O]): 0.360 to 0.600

In a preferred embodiment, the properties that the glass compositionaccording to the present invention can have, specifically the elasticmodulus, density, and temperature properties, are as follows.

The Young's modulus is preferably 85 GPa or more, 86 GPa or more, oreven 86.5 GPa or more. The density is preferably 2.565 g/cm³ or less,2.550 g/cm³ or less, or even 2.546 g/cm³ or less. The specific elasticmodulus is preferably 34×10⁶ Nm/kg or more or even 34.5×10⁶ Nm/kg ormore. The specific elastic modulus is a value calculated by dividing theYoung's modulus by the density. Such a high specific elastic modulus asdescribed above is advantageous in terms of preventing deflection of arotating substrate. In a preferred embodiment, the glass compositionaccording to the present invention can have both a high specific elasticmodulus as described above and a low density as described above.

In a preferred embodiment, the glass composition according to thepresent invention has the following properties.

Young's modulus: 85 GPa or moreDensity: 2.565 g/cm³ or lessSpecific elastic modulus: 34×10⁶ Nm/kg or more

The devitrification temperature TL is preferably 1110° C. or lower,1105° C. or lower, or even 1100° C. or lower. A shaping temperature T4is preferably 1020° C. or higher or even 1040° C. or higher. Adifference ΔT (ΔT=T4−TL) determined by subtracting the devitrificationtemperature TL from the shaping temperature T4 is preferably greaterthan 0, more preferably 5° C. or higher, even more preferably 10° C. orhigher, much more preferably 15° C. or higher, and particularlypreferably 18° C. or higher. Here, the shaping temperature T4 is atemperature at which the viscosity measured by a platinum ball-drawingmethod is 104 dPa·s. The devitrification temperature TL is the highesttemperature at which devitrification was observed in glass taken out ofa temperature-gradient electric furnace, the glass being obtained byholding a crushed glass specimen in the furnace for 2 hours. In apreferred embodiment, the glass composition according to the presentinvention can have a low devitrification temperature as described aboveand a positive difference ΔT.

A preferred strain point is less than 530° C. A glass having such aproperty temperature has a glass-transition point lower than 580° C. oreven lower than 570° C. This property is determined to make glassshaping and post-shaping cooling much easier.

A preferred linear thermal expansion coefficient is 80 to 88×10⁻⁷/° C.Here, the linear thermal expansion coefficient refers to the averagelinear thermal expansion coefficient in the temperature range of 50 to350° C. This linear thermal expansion coefficient value is lower thanthat of common soda-lime glass used, for example, for windows ofbuildings and vehicles. A lower linear thermal expansion coefficient isadvantageous in terms of reducing residual strain, and can reducewarping of a glass sheet and improve cutting stability, for example,when the glass sheet is produced by a float process.

As can be easily understood from the above temperature properties, theglass composition according to the present invention is suitable formass production by a float process. By the float process, a glass sheetcalled float glass is produced from the glass composition according tothe present invention. As is well known, the float process includes:melting glass raw materials in a melting furnace; and introducing themolten glass raw materials into a float bath to shape the molten glassraw materials into a glass sheet on molten tin in the float bath. In oneembodiment of the present invention, float glass is produced bypreparing glass raw materials so that a glass composition forming theresulting glass sheet will have the above desirable composition. Thefloat glass is shaped with one principal surface in contact with moltentin in a float bath, and the tin spreads over the principal surface.Accordingly, one principal surface, called a bottom surface, of thefloat glass has a surface layer formed of tin spread thereon. The otherprincipal surface, called a top surface, does not have such a surfacelayer. From another perspective, in the float glass, the concentrationof tin on one principal surface is higher than that on the otherprincipal surface.

The glass sheet may be a chemically strengthened glass sheet. As is wellknown, chemical strengthening is a treatment in which compressive stressis introduced in a surface of glass by substituting alkali ionscontained in glass with other alkali ions having a larger ionic radius,for example, by substituting lithium ions with sodium ions or sodiumions with potassium ions. Chemical strengthening of the glass sheet iscommonly performed by bringing the glass sheet into contact with amolten salt including alkali ions. The molten salt is, for example,potassium nitrate or a salt mixture of potassium nitrate and sodiumnitrate. When the molten salt used includes potassium nitrate alone, itis appropriate that the molten salt be at a temperature of about 460° C.to 500° C. in view of thermal decomposition of potassium nitrate and thethermal resistance of the glass. It is appropriate that the time duringwhich the glass and the molten salt are in contact with each other be,for example, 4 hours to 12 hours.

The chemically strengthened glass sheet is particularly suitable for asubstrate for an information recording medium. The glass sheet accordingto the present invention can also be used in other applications such ascover glasses of displays and solar cells.

EXAMPLES

Hereinafter, the present invention will be described in more detailusing specific examples. The examples given below are not intended tolimit the present invention, either.

Batches were prepared to give compositions shown in Table 1 usingsilica, alumina, lithium carbonate, sodium carbonate, magnesium oxide,calcium carbonate, potassium carbonate, etc. which are common glass rawmaterials. Each of the prepared batches was put in a platinum crucible,held at 1580° C. for 4 hours, and then poured onto an iron plate. Thisglass was held in an electric furnace at 650° C. for 30 minutes, afterwhich the furnace was turned off to cool the glass to room temperature.A glass specimen was thus obtained. Properties of thus-obtained glassspecimens were measured by the following methods. Table 1 shows theresults.

[Density and Young's Modulus]

Plate-shaped samples having dimensions of 25×25×5 mm were fabricated bycutting the glass specimens and mirror-polishing every surface thereof.The density of each sample was measured by Archimedes' principle. TheYoung's modulus of each sample was measured according to the ultrasonicpulse method in JIS R 1602-1995. Specifically, each sample used in theabove density measurement was used to measure, for longitudinal andtransverse waves, the sound speed at which an ultrasonic pulsepropagates. The sound speeds and the above density were substituted inthe formula defined in JIS R 1602-1995 to calculate the Young's modulusE. The propagation speeds were evaluated using an ultrasonic thicknessgage MODEL 25DL PLUS manufactured by Olympus Corporation by dividing thetime required by a 20 kHz ultrasonic pulse to propagate in the thicknessdirection of the sample, be reflected, and then come back by thepropagation distance (twice the thickness of the sample).

[Glass-Transition Point and Linear Thermal Expansion Coefficient]

A cylindrical specimen having a diameter of 5 mm and a length of 18 mmwas fabricated from each glass specimen. The cylindrical specimen washeated at 5° C./minute using a TMA apparatus to measure a thermalexpansion curve. From this curve, the glass-transition point and theaverage linear thermal expansion coefficient in the temperature range of50 to 350° C. were obtained.

[Measurement of Devitrification Temperature TL]

Each glass specimen was crushed into particles, which were sieved toobtain particles that pass through a sieve having an opening size of2.83 mm and are retained on a sieve having an opening size of 1.00 mm.These particles were washed to remove fine powder thereon, followed bydrying to prepare a sample for devitrification temperature measurement.An amount of 25 g of the sample for devitrification temperaturemeasurement was put in a platinum boat (a lidless rectangular platinumcontainer) to have an approximately uniform thickness, held in atemperature-gradient furnace for 2 hours, and then taken out of thefurnace. The highest temperature at which devitrification was observedin the glass was employed as the devitrification temperature of thesample.

[Measurement of Shaping Temperature T4]

The viscosity was measured by a platinum ball-drawing method, and atemperature at which the thus-measured viscosity was 104 dPa·s wasemployed as the shaping temperature T4.

The specimens 1 to 13 have a Young's modulus of 85 GPa or more, adensity of 2.565 g/cm³ or less, and a specific elastic modulus of 34×10⁶Nm/kg or more. The specimens 1 to 12 have a devitrification temperatureof 1110° C. or lower while the specimen 13 has a devitrificationtemperature higher than 1110° C.

TABLE 1 Specimen No. 1 2 3 4 5 6 7 SiO₂ 61.52 59.52 57.52 61.52 61.5259.52 59.52 Al₂O₃ 12.21 13.21 14.21 12.87 13.45 13.54 14.15 MgO 2.542.73 2.92 2.42 2.31 2.67 2.55 CaO 7.63 8.19 8.75 7.26 6.93 8.01 7.66Li₂O 8.28 8.41 8.54 8.20 8.12 8.37 8.29 Na₂O 7.63 7.75 7.87 7.56 7.497.71 7.64 K₂O 0.18 0.18 0.18 0.18 0.18 0.18 0.18 R′O = MgO + 10.17 10.9211.67 9.68 9.25 10.68 10.22 CaO R₂O = Li₂O + 16.10 16.35 16.60 15.9315.79 16.27 16.11 Na₂O + K₂O Al₂O₃ − R₂O −3.88 −3.13 −2.38 −3.06 −2.34−2.72 −1.96 Al₂O₃/(Al₂O₃ + 0.166 0.182 0.198 0.173 0.179 0.185 0.192SiO₂) (R₂O + R′O)/ 0.683 0.674 0.665 0.666 0.651 0.665 0.650 (R₂O +R′O + Al₂O₃) R₂O/(R₂O + R′O) 0.613 0.599 0.587 0.622 0.631 0.604 0.612CaO/R′O 0.750 0.750 0.750 0.750 0.750 0.750 0.750 Na₂O/(Na₂O + 0.4800.480 0.480 0.480 0.480 0.480 0.480 Li₂O) Glass-transition 544 548 553550 559 553 556 point (° C.) Linear thermal 84 85 86 82 81 84 83expansion coefficient (10⁻⁷/° C.) Density (g/cm³) 2.509 2.523 2.5362.505 2.502 2.521 2.517 Young's modulus 86.7 87.8 88.7 86.9 86.9 88.187.8 (GPa) Specific elastic 34.6 34.8 35.0 34.7 34.7 35.0 34.9 modulus(10⁶ Nm/kg) Devitrification 1023 1029 1031 1061 1080 1054 1072temperature TL (° C.) Shaping 1051 1041 1075 1080 1103 1073 1105temperature T4 (° C.) ΔT = T4 − TL 28 12 44 19 23 19 33 (° C.) SpecimenNo. 8 9 10 11 12 13* SiO₂ 57.52 57.52 56.52 56.52 56.52 56.52 Al₂O₃14.85 15.49 14.55 14.71 15.20 16.60 MgO 2.80 2.68 3.04 3.01 2.92 2.66CaO 8.39 8.03 9.13 9.03 8.76 7.98 Li₂O 8.46 8.38 8.63 8.61 8.55 8.36Na₂O 7.80 7.72 7.95 7.93 7.88 7.71 K₂O 0.18 0.18 0.18 0.18 0.18 0.18 R′O= MgO + 11.19 10.71 12.17 12.05 11.68 10.63 CaO R₂O = Li₂O + 16.44 16.2816.76 16.72 16.60 16.25 Na₂O + K₂O Al₂O₃ − R₂O −1.58 −0.78 −2.21 −2.01−1.40 0.35 Al₂O₃/(Al₂O₃ + 0.205 0.212 0.205 0.207 0.212 0.227 SiO₂)(R₂O + R′O)/ 0.650 0.635 0.665 0.662 0.650 0.618 (R₂O + R′O + Al₂O₃)R₂O/(R₂O + R′O) 0.595 0.603 0.579 0.581 0.587 0.604 CaO/R′O 0.750 0.7500.750 0.750 0.750 0.750 Na₂O/(Na₂O + 0.480 0.480 0.480 0.480 0.480 0.480Li₂O) Glass-transition 560 568 555 556 560 578 point (° C.) Linearthermal 84 83 85 85 85 82 expansion coefficient (10⁻⁷/° C.) Density(g/cm³) 2.531 2.528 2.542 2.542 2.539 2.531 Young's modulus 88.6 88.589.3 89.3 89.2 89.4 (GPa) Specific elastic 35.0 35.0 35.1 35.1 35.1 35.3modulus (10⁶ Nm/kg) Devitrification 1074 1105 1046 1039 1063 1127temperature TL (° C.) Shaping 1122 1173 1070 1078 1119 1017 temperatureT4 (° C.) ΔT = T4 − TL 48 68 24 39 56 −110 (° C.) *No. 13 is acomparative example. The contents of the components are in mol %. Thesymbol [ ] used to indicate the contents is omitted in the table.

1. A glass composition comprising, as components, in mol %: 50 to 70%SiO₂; 10 to 20% Al₂O₃; 2 to 5% MgO; 3 to 15% CaO; 3 to 15% Li₂O; 3 to15% Na₂O; and 0 to 5% K₂O, wherein when a content of a component X inmol % is expressed as [X], [Al₂O₃]— [R₂O] is less than −0.5%, where[R₂O] is the sum of [Li₂O], [Na₂O], and [K₂O].
 2. The glass compositionaccording to claim 1, wherein [SiO₂] is 52 to 68%, [R′O] is 5 to 15%,where [R′O] is the sum of [MgO] and [CaO], and [R₂O] is 10 to 20%. 3.The glass composition according to claim 1, comprising, in mol %: 53 to67% SiO₂; 10 to 19% Al₂O₃; 2 to 4% MgO; 4 to 12% CaO; 5 to 12% Li₂O; 5to 12% Na₂O; and 0 to 3% K₂O.
 4. The glass composition according toclaim 3, comprising, in mol %: 55 to 65% SiO₂; 11 to 18% Al₂O₃; 2 to3.5% MgO; 5 to 11% CaO; 7 to 10% Li₂O; 7 to 10% Na₂O; and 0 to 2% K₂O.5. The glass composition according to claim 1, comprising, in mol %: 55to 64% SiO₂ (excluding 64%); 11 to 16% Al₂O₃; 2 to 3.5% MgO; 5 to 11%CaO (excluding 5%); 7.7 to 10% Li₂O (excluding 7.7% and 10%); 7 to 9%Na₂O (excluding 9%); and 0 to 2% K₂O, wherein [R′O] is 8 to 13%, where[R′O] is the sum of [MgO] and [CaO], and [R₂O] is 15 to 18%.
 6. Theglass composition according to claim 5, wherein [Al₂O₃]/([Al₂O₃]+[SiO₂])is 0.140 to 0.220, ([R₂O]+[R′O])/([R₂O]+[R′O]+[Al₂O₃]) is 0.610 to0.700, and [R₂O]/([R₂O]+[R′O]) is 0.550 to 0.630.
 7. The glasscomposition according to claim 5, wherein [CaO]/([CaO]+[MgO]) is 0.650to 0.850, and [Na₂O]/([Na₂O]+[Li₂O]) is 0.360 to 0.600.
 8. The glasscomposition according to claim 1, wherein [SiO₂] is 55 to 62%.
 9. Theglass composition according to claim 1, wherein [MgO] is less than 3%.10. The glass composition according to claim 1, further comprising, inmol %: 0 to 0.2% TiO₂; and 0 to 0.2% ZrO₂, wherein [TiO₂]+[ZrO₂] is 0.2%or less.
 11. The glass composition according to claim 1, furthercomprising, in mol %: 0 to 0.07% SnO₂; and 0 to 0.07% CeO₂, wherein[SnO₂]+[CeO₂] is 0.07% or less, and the glass composition issubstantially free of P₂O₅, B₂O₃, SrO, BaO, Nb₂O₅, a lanthanide oxide,Y₂O₃, Ta₂O₅, and F.
 12. The glass composition according to claim 1,wherein a Young's modulus is 85 GPa or more, a density is 2.565 g/cm³ orless, and a specific modulus is 34×10⁶ Nm/kg or more.
 13. The glasscomposition according to claim 1, wherein a difference ΔT determined bysubtracting a devitrification temperature TL from a shaping temperatureT4 is greater than
 0. 14. A glass sheet formed of the glass compositionaccording to claim
 1. 15. The glass sheet according to claim 14 beingfloat glass.
 16. A substrate for an information recording medium,comprising the glass sheet according to claim
 14. 17. A method forproducing a glass sheet, comprising: melting glass raw materials; andshaping the molten glass raw materials into a glass sheet by a floatprocess, wherein the glass raw materials are prepared so that the glasssheet is formed of the glass composition according to claim 1.